Solar & Lunar Eclipse Geometry Simulator
Move the Moon through its real tilted orbit and see exactly why eclipses need both the right phase AND the right node crossing.
How to use: drag the Moon's orbital phase and its tilt relative to Earth's orbital plane (the ecliptic). An eclipse only happens when the Moon is both at new/full phase (syzygy) AND crossing the ecliptic plane (a node) at that same time.
Challenge 1: cause a solar eclipse
Bring the Moon to a new moon phase while it is also near a node (height above ecliptic close to 0) so the Moon's shadow reaches Earth.
Challenge 2: cause an annular eclipse
Align a solar eclipse as above, but with the Moon near apogee (far distance) so it looks too small to fully cover the Sun.
About the Solar & Lunar Eclipse Geometry Simulator
Free solar & lunar eclipse geometry simulator. Move the Moon through its real tilted orbit and see exactly why eclipses need both the right phase AND the right node crossing. Drag, change the sliders and see the result live. No sign-up, works on phone and computer. Built for astronomy, the solar & lunar eclipse geometry simulator runs instantly in your browser: change a setting or drag an object and the result updates at once, so you learn by trying things out rather than only reading about them.
Move the Moon through its real tilted orbit and see exactly why eclipses need both the right phase AND the right node crossing. Use it to explore astronomy ideas at your own pace, then check what you found against the key ideas further down this page.
How to use the Solar & Lunar Eclipse Geometry Simulator
- Use the controls to change Moon's position in orbit (days since new moon), Orbital node offset (days from a node), Moon distance (perigee → apogee). The simulation reacts instantly.
- Press "▶ Animate orbit", "🔭 Jump to next eclipse", "Reset to defaults", "📄 Lab Report" to start, reset or change what is happening.
- Where you see a glowing handle, object, weight or atom, drag it with your mouse or finger. Everything responds in real time.
- Watch the readouts and graphs update as you experiment, and compare what you see with the key ideas below.
Things to try
- Set phase to new moon (0 days) with the node offset at 0 and watch for a solar eclipse.
- Keep the same alignment but drag the distance slider to apogee and watch it become annular.
- Set phase to full moon (about 14.75 days) with node offset near 0 for a lunar eclipse.
- Move only the node offset slider and watch how quickly the eclipse turns off as the Moon leaves the ecliptic plane.
Key ideas you can learn
- An eclipse needs syzygy: the Sun, Earth and Moon nearly in a straight line, which only happens at new moon (solar eclipse) or full moon (lunar eclipse).
- The Moon's orbit is tilted about 5.14° to Earth's orbital plane (the ecliptic), so most new and full moons pass above or below the Sun-Earth line and no eclipse occurs.
- An eclipse also needs the Moon to be near a node - where its tilted orbit crosses the ecliptic plane - at the same time as syzygy.
- Because the Moon's orbit is an ellipse, its distance (and apparent size) varies; near apogee it looks too small to fully cover the Sun, producing an annular ('ring of fire') eclipse instead of a total one.
Where this is used in the real world
Astronomers predict eclipses centuries in advance using exactly this combination of orbital phase, node timing and distance, which is also why total solar eclipses are visible from only a narrow path on Earth.
Who is this simulation for?
Students, teachers and curious learners of all ages.
For teachers: project it on the board, let students predict what will happen, then run it together. For students: change one thing at a time and write down what changes.
Frequently asked questions
Why doesn't a solar eclipse happen every single new moon?
The Moon's orbit is tilted about 5.14° to the ecliptic, so on most months the new moon passes above or below the Sun as seen from Earth; only when a new moon coincides with the Moon being near one of its two orbital nodes does its shadow actually reach Earth.
What determines whether a solar eclipse is total or annular?
It depends on the Moon's distance from Earth at that moment: near perigee the Moon's apparent size exceeds the Sun's and fully covers it (total), while near apogee the Moon looks smaller than the Sun and leaves a bright ring visible around it (annular).
Is the Solar & Lunar Eclipse Geometry Simulator free to use?
Yes. It is completely free, with no signup, no download and no ads inside the simulation. It runs in your web browser.
Does the Solar & Lunar Eclipse Geometry Simulator work on a phone or tablet?
Yes. It uses touch as well as the mouse, so you can drag objects with your finger. A larger screen makes the controls easier to see.